Communications cables which are strung between poles or those which are buried in the ground are subjected to abuse such as, for example, attack by rodents, mechanical abrasion and crushing. Attacks by gophers on buried cable and by squirrels on aerial cable have been a continuing concern. Gophers, for example, have been shown to exert biting pressures as high as 18,000 psi. Cables having an outside diameter below a critical size of about 0.75 inch in diameter are more apt to be damaged than larger cables because the animals can bite directly down on them. For larger size cables, only a scraping or raking action takes place. In fact, on cables exceeding about 2 inches in diameter, gopher attack is rarely observed.
It has been found that with limited exceptions, the only way to protect directly exposed cables from rodent attack is to wrap them in a metallic shield. A longitudinally applied shield, if otherwise suitable, would be economically preferable from a manufacturing standpoint. For cables above the critical size, the use of a corrugated shield having a longitudinally overlapped seam generally has provided sufficient protection. However, in the smaller sizes, such a shield arrangement had led to failures. Rodents have been able to encompass the cable with their teeth and pull open the seam.
Both buried and aerial cables also are damaged by lightning strikes. Thermal damage, that is burning, charring and melting of the sheath components, is caused by the heating effects of the lightning arc and a current being carried to ground by the metallic members of the core or sheath. In buried cables, a second mode of damage is mechanical, causing crushing and distortion of the sheath. This results from an explosive impact, sometimes called a steamhammer effect, which is caused by the instantaneous vaporization of water in the earth in a lightning channel to the cable.
The prior art abounds with patents relating to metallic sheath systems for copper core cables such as one comprising an aluminum shield enclosed by a carbon steel shield with each having a longitudinal seam. This sheath system offers protection from mechanical damage, electromagnetic interference and lightning and its cost is quite low because it is made in a single pass at relatively high line speeds. However, the use of a shield which is made of carbon steel occasionally has resulted in long term failures, even in cables larger than 0.75 inch. Failure may occur because the underlying steel shield may become exposed when rodents violate the jacket. Once exposed, the steel shield, which withstands the initial attack by rodents, corrodes readily. This renders it ineffective for general mechanical protection and for protection from any subsequent rodent attack. In this regard, it should be pointed out that gophers are territorial animals which repeatedly return to areas previously occupied by them. Therefore, it is not uncommon to experience secondary attacks in the same location along a cable. These cables may fail also because of the presence of a longitudinal seam formed in a shield which is made of a steel having mechanical properties which are not sufficient to cause the shield to be protected from rodent abuse.
The prior art also includes a cable which is enclosed in a copper inner shield and a relatively thin steel outer shield which is bonded to an outer plastic jacket. Because of the relatively thin outer shield, the plastic jacket can be removed easily to facilitate grounding without damaging the inner shield. Ease of jacket removal, however, is not a characteristic which is desired for a cable that is exposed to rodents.
Lately, lightguide fiber cables have made inroads into the communications cable market. They too are subject to rodent attack. Inasmuch as lightguide fiber cables fall into a range of about 0.5 to 0.8 inch in diameter, the use of longitudinal overlapped seams has been in question. A prior art lightguide cable sheath system which offers rodent protection comprises two helically wrapped, non-corrugated stainless steel shielding tapes enclosed in a plastic jacket. However, it has several shortcomings. It is very expensive to manufacture because of low line speeds and the complex machinery required to wrap the tapes helically about a core, and because the taping and jacketing have to be accomplished in two separate operations. Although metallic conductors are not used for transmission in lightguide fiber cables, metallic members are commonly used in the sheath system, for example. The protection offered against lightning strikes is affected adversely because of the relatively low conductivity of stainless steel and the relatively low impact resistance of helically applied, flat tapes. Rodent and lightning protection are perhaps even more critical for a lightguide fiber cable due to its relatively high capacity and the fragility of the glass fibers. Also, lightguide cable is exposed to typical mechanical hazards such as abrasion and crushing, for example, during installation.
Seemingly, the prior art is devoid of an economical sheath system which provides protection against rodents and lightning, as well as against mechanical hazards, particularly for small size cables such as might be used in lightguide communications. What is desired is a cable structure which resists degradation by requiring rodents to remove laboriously each elemental piece of jacket material from a tough, durable metallic shield.